1. Field of the Invention
This invention relates to compositions and processes for treating metal surfaces with acidic aqueous compositions for forming conversion coatings on the metals; the conversion coatings provide excellent bases for subsequent painting. The invention is well suited to treating iron and steel, galvanized iron and steel, zinc and those of its alloys that contain at least 50 atomic percent zinc, and aluminum and its alloys that contain at least 50 atomic percent aluminum. Preferably the surface treated is predominantly ferrous; most preferably the surface treated is cold rolled steel.
2. Statement of Related Art
A very wide variety of materials have been taught in the prior art for the general purposes of the present invention, but most of them contain hexavalent chromium which is environmentally undesirable. One object of this invention is to avoid any substantial use of hexavalent chromium and other materials such as ferricyanide that have been identified as environmentally damaging.